Well, lemme ask ... what's the advantage to having an underage gymnast? Wouldn't a 16-year-old who has trained longer and is more developed muscularly have an advantage over a young child? ... I'm not quite making the connection here with younger (than 16) = an advantage.
I think it's a size/weight advantage. Otherwise, yeah, I think the age requirement is pretty much arbitrary. Then again, there has to be a minimum age requirement, doesn't there?